MHC antigens were first characterized as proteins that played a major role in transplantation reactions. Rejection is mediated by T cells reacting to the histocompatibility antigens on the surface of implanted tissues, and the largest group of these antigens is the major histocompatibility antigens (MHC). These proteins are expressed on the surface of all higher vertebrates and are called H-2 antigens in mice (for histocompatibility-2 antigens) and HLA antigens (for human leukocyte antigens) in human cells.
The MHC proteins serve a vital role in T cell stimulation. Antigen presenting cells (often dendritic cells) display peptides that are the degradation products of foreign proteins on the cell surface on the MHC. In the presence of a co-stimulatory signal, the T cell becomes activated, and will act on a target cell that also displays that same peptide/MHC complex. For example, a stimulated T helper cell will target a macrophage displaying an antigen in conjunction with its MHC, or a cytotoxic T cell (CTL) will act on a virally infected cell displaying foreign viral peptides.
MHC proteins are of two classes, I and II. The class I MHC proteins are heterodimers of two proteins, the α chain, which is a transmembrane protein encoded by the MHC1 gene, and the β2 microblogulin chain, which is a small extracellular protein that is encoded by a gene that does not lie within the MHC gene cluster. The α chain folds into three globular domains and when the β2 microglobulin chain is associated, the globular structure complex is similar to an antibody complex. The foreign peptides are presented on the two most N-terminal domains which are also the most variable. Class II MHC proteins are also heterodimers, but the heterodimers comprise two transmembrane proteins encoded by genes within the MHC complex. The class I MHC:antigen complex interacts with cytotoxic T cells while the class II MHC presents antigens to helper T cells. In addition, class I MHC proteins tend to be expressed in nearly all nucleated cells and platelets (and red blood cells in mice) while class II MHC protein are more selectively expressed. Typically, class II MHC proteins are expressed on B cells, some macrophage and monocytes, Langerhans cells, and dendritic cells.
The class I HLA gene cluster in humans comprises three major loci, B, C and A, as well as several minor loci. The class II HLA cluster also comprises three major loci, DP, DQ and DR, and both the class I and class II gene clusters are polymorphic, in that there are several different alleles of both the class I and II genes within the population. There are also several accessory proteins that play a role in HLA functioning as well. The Tap1 and Tap2 subunits are parts of the TAP transporter complex that is essential in loading peptide antigens on to the class I HLA complexes, and the LMP2 and LMP7 proteosome subunits play roles in the proteolytic degradation of antigens into peptides for display on the HLA. Reduction in LMP7 has been shown to reduce the amount of MHC class I at the cell surface, perhaps through a lack of stabilization (see Fehling et al (1999) Science 265:1234-1237). In addition to TAP and LMP, there is the tapasin gene, whose product forms a bridge between the TAP complex and the HLA class I chains and enhances peptide loading. Reduction in tapasin results in cells with impaired MHC class I assembly, reduced cell surface expression of the MHC class I and impaired immune responses (see Grandea et al (2000) Immunity vol 13:213-222 and Garbi et al (2000) Nat Immunol 1:234-238).
Regulation of class I expression is generally at the transcriptional level, and several stimuli such as viral infection etc. can cause a change in transcription. The class I genes are down-regulated in some specific tissues, and the source of this down-regulation seems to be within the promoter and 3′ intergenic sequences (see Cohen et al (2009) PLos ONE 4(8): e6748). There is also evidence that microRNAs are capable of regulating some class I MHC genes (see Zhu et al, (2010) Am. J. Obstet Gynecol 202(6):592).
Regulation of class II MHC expression is dependent upon the activity of the MHCII enhanceosome complex. The enhanceosome components (one of the most highly studied components of the enhanceosome complex is the RFX5 gene product (see Villard et al (2000) MCB 20(10): 3364-3376)) are nearly universally expressed and expression of these components does not seem to control the tissue specific expression of MHC class II genes or their IFN-γ induced up-regulation. Instead, it appears that a protein known as CIITA (class II transactivator) which is a non-DNA binding protein, serves as a master control factor for MCHII expression. In contrast to the other enhanceosome members, CIITA does exhibit tissue specific expression, is up-regulated by IFN-γ, and has been shown to be inhibited by several bacteria and viruses which can cause a down regulation of MHC class II expression (thought to be part of a bacterial attempt to evade immune surveillance (see Leibund Gut-Landmann et al (2004) Eur. J. Immunol 34:1513-1525)).
Regulation of the class I or II genes can be disrupted in the presence of some tumors and such disruption can have consequences on the prognosis of the patients. For example, in some melanomas, an observed reduction in Tap 1, Tap 2 and HLA class I antigens was found to be more common in metastatic melanomas (P<0.05) than in primary tumors (see Kageshita et al (1999) Am Jour of Pathol 154(3):745-754).
In humans, susceptibility to several diseases is suspected to be tied to HLA haplotype. These diseases include Addison's disease, ankylosing spondylitis, Behçet's disease, Buerger's disease, celiac disease, chronic active hepatitis, Graves' disease, juvenile rheumatoid arthritis, psoriasis, psoriatic arthritis, rheumatoid arthritis, Sjögren syndrome, and lupus erythematosus, among others.
HLA haplotype also plays a major role in transplant rejection. The acute phase of transplant rejection can occur within about 1-3 weeks and usually involves the action of host T lymphocytes on donor tissues due to sensitization of the host system to the donor class I and class II HLA molecules. In most cases, the triggering antigens are the class I HLAs. For best success, donors are typed for HLA haplotype and matched to the patient recipient as completely as possible. But donation even between family members, which can share a high percentage of HLA haplotype identity, is still often not successful. Thus, in order to preserve the graft tissue within the recipient, the patient often must be subjected to profound immunosuppressive therapy to prevent rejection. Such therapy can lead to complications and significant morbidities due to opportunistic infections that the patient may have difficulty overcoming.
Cell therapy is a specialized type of transplant wherein cells of a certain type (e.g. T cells reactive to a tumor antigen or B cells) are given to a recipient. Cell therapy can be done with cells that are either autologous (derived from the recipient) or allogenic (derived from a donor) and the cells may be immature cells such as stem cells, or completely mature and functional cells such as T cells. In fact, in some diseases such certain cancers, T cells may be manipulated ex vivo to increase their avidity for certain tumor antigens, expanded and then introduced into the patient suffering from that cancer type in an attempt to eradicate the tumor. This is particularly useful when the endogenous T cell response is suppressed by the tumor itself. However, the same caveats apply for cell therapy as apply for more well known solid organ transplants in regards to rejection. Donor T cells express class I HLA antigens and thus are capable of eliciting a rejection response from the recipient's endogenous immune system.
Thus, there remains a need for compositions and methods for the manipulation of HLA genes and gene expression in cells.